Process for the treatment of metal surfaces prior to enameling



United States Patent 4 Claims. (Cl. 117-53 ABSTRACT OF THE DISCLOSURE Aqueous solutions of mixtures for the total removal of iron salt from iron and steel sheet formed during etching prior to passivation in preparation for vitreous enameling of the sheet. The active ingredients of'the mixture are an alkali together with a lower aliphatic alkanolamine, wherein the alkali constitutes 25 to 50 weight percent of the mixture in a 1 to 5 weight percent solution. It also is opportune to incorporate in the solution an aminoca-rboxylic acid which amounts to up to 20 weight percent, based upon the total amount of said acid taken together with said alkali and said alkanolamine. These solutions remove all iron salts present and yield a clean and silver-white surface thus ensuring perfect conditions for enameling.

The invention relates to the treatment of metal sheets preparatory to the application of vitreous enamel, particularly iron and steel sheets. More particularly, it relates to a treatment with materials which do not form noxious and poisonous gases, yielding clean and silver-white metal surfaces.

It is known that the surface pretreatment of sheet iron and steel, especially of preformed sheets, greatly influences the ensuing enameling. Therefore, the sheets not only are carefully degreased, but other intermediate treatment steps are carried out, such as etching and, if required, neutralization. Acid treatments with sulfuric or hydrochloric acid necessarily leads to the formation of iron salts and also of the salts of those metals which are present in the steel sheet as deliberate or accidental alloying components. For this reason, etching is followed by a rinsing step wherein not only the acid film, but also the salts formed are to be removed. Since basic salts readily form which frequently adhere rather tenaciously to the sheet and can be removed only with difficulty especially from the pores of the sheets,

a plurality of rinsing baths often is employed, and. each r of these baths is adjusted to a suitable pH value.

However, faults or flaws in the vitreous enamel coatings even occur when several rinse baths are employed. These flaws again are the result of unremoved salt residues. Therefore, it has become the practice to add a further treatment, after rinsing, with an alkaline sodium cyanide solution. The remaining salt thereby is bound in complex form, and these complexes can be removed by further rinsing and passivation treatments of the metal sheets. Whereas this process largely eliminates waste products, it has the grave disadvantage that the cyanide baths must be kept under strong surveillance in order to obviate a carryover of residual acid and thus the formation of hydrocyanic acid. For the same reason, cyanide-containing solutions cannot be used in enamel spray processes since even the influence of carbon dioxide present in the atmosphere tends to form hydrocyanic acid.

It now has been found that these drawbacks can be avoided by the process according to the invention. This process for the pretreatment of metal surfaces preparatory to vitreous enameling and especially for the removal of salt residues therefrom after treatment with acidic solu-' tions is characterized by a treatment of the metal surfaces with solutions of alkanolamines containing at least 3 alkanol groups in the molecule and alkali.

As alkali, sodiumor potassium hydroxide, soda, potassium carbonate, or mixtures thereof, are suited. The pH of the solutions always is above 10. Especially readily available alkanolamines of the type named above are trialkanolamines having an alkyl radical of 2 to 4 carbon atoms, especially triethanolamine and polyalkanol compounds, such as N,N,N',N'-tetrakis (Z-hydroxypropyl) ethylenediamine. The range of activity of solutions of this kind is particularly surprising as satisfactory metal surfaces cannot be obtained with the use of solutions which contain merely alkali and complex-forming compounds, e.g., gluconates, anhydrous phosphates, oxalates or tartaric acid.

It frequently is opportune to add to the solutions withwhich the process according to the invention is carried out, aminopolycarboxylic acids or their salts, in addition to the components named above. Suitable aminocarboxylic acids especially are readily accessible acids such as nitrilotriacetic acid, ethylenediaminotriacetic acid, and ethylenediaminotetraacetic acid, and their water-soluble alkali metal salts. The advantage of their addition consists particularly in the independence of such compounds from the hardness of the water employed. However, solutions containing solely aminopolycarboxylic acids and alkali do not produce satisfactory results. Furthermore, it is advantageous to have the individual component present in the solutions in definite quantities. The concentration of the mixture in the aqueous solutions themselves is 1 to 5 weight percent. Generally, it is of advantage to prepare concentrates and to dilute them just prior to application. It has been found opportune to hold the alkali content, calculated on the total components exclusive of water, above percent, and preferably between and weight percent. When aminopolycarboxylic acids are added, their content, also calculated on the total components exclusive of water, should amount to up to 20 weight percent. The remainder is alkanolamine, also exclusive of water. As stated above with respect to prior art, the rinsing baths following the acid pretreatment, such as etching or degreasing, frequently are adjusted to a given pH value in order to effect neutralization. When the process according to the invention is employed, it is opportune to use an alkaline-reacting intermediary rinse bath. In certain instances it is advantageous toadd even to such an intermediary rinse bath alkanolamine and, if desired, aminopolycarboxylic acid. The working life of the solution according to the invention which, in contrast to cyanide baths, can be employed under any conditions, thereby is lengthened considerably. The intermediary bath as well as the ensuing treatment according to the invention can be carried out at temperatures ranging from 10 to 90 C. preferably 20 to C. and especially at room temperature. The treatment in the intermediary bath as well as the ensuing treatment according to the invention can be carried out for a period of 1 to 15 minutes, preferably 2 to 10 minutes.

The invention now will be further explained by the following examples. However, it should be understood that these are given merely by way of illustration, and not of limitation, and that numerous changes may be made in the details without departing from the spirit and the scope of the invention as hereinafter claimed.

Example 1 Sheet iron was well degreased, etched with 10% aqueous HCl containing 6% ferrochloride, and rinsed. They then were immersed for 2 minutes in a 2% aqueous solution of a mixture consisting of 40 parts by weight NaOH, 40 parts by weight triethanolamine, and 20 parts by weight tetrasodiumethylene diamine tetraacetate. Passivation was carried out immediately thereafter, without an intermediary rinse, in a customary passivating bath (1% aqueous solution composed of 60 parts by weight soda and 40 parts by weight trisodium phosphate; temperature 80 C). The sheets thus obtained were silver-white and could be enameled without exhibiting faults or flaws.

Upon a like treatment but without triethanolamine, sheets were obtained whose surface still contained a part of the iron salts. These sheets had a greenish-brownish color and did not lend themselves to flawless enameling.

Example 2 Well degreased steel sheets were etched in a bath of 10% aqueous sulfuric acid containing 50 g./l. iron for 10 minutes at a bath temperature of 60 C. by immersion, and then rinsed with water. Thereafter the sheets were put into an alkaline intermediary bath at room temperature for minutes. This bath consisted of a 2% aqueous solution of equal parts by weight NaOH, soda and triethanolamine. Good agitation was provided. Afterward the sheets were dipped into a 4% aqueous solution consisting of 30 parts by weight NaOH, 5 parts by weight ethylenediamine tetraacetic acid and 65 parts by weight triethanolamine at a temperature of 25 C. and left therein for 2 minutes. The sheets thereafter had a silver color which did not change during and after the passivating treatment as described in Example 1. These sheets were enameled without flaws.

Equally good results were obtained when the treatment was carried out by spraying in lieu of immersion.

Example 3 Degreased iron sheets were etched with 15% aqueous HCl and rinsed with water. They then were immersed for 5 minutes in a 3% aqueous solution of 60 C. containing equal parts by weight NaOH, triisopropanolamine and N,N,N,N' tetrakis (Z-hydroxypropyl)-ethylenediamine, and then passivated, without intermediary rinse, in a 0.5% aqueous solution at 80-90 C. containing equal parts by weight soda, phosphate and borax.

The silver-white surface of the metal sheet contained no residual iron compounds so that the conditions for a flawless enameling were present.

4 Example 4 Cold rolled steel sheets were degreased and etched with a 15% aqueous HCl solution containing iron, rinsed, and immersed for 5 minutes in a 3% aqueous solution at 25 C., containing 30 parts by weight NaOH, 30 parts by weight triethanolamine, 30 parts by weight triisopropanolamine and 10 parts by weight tetrasodiumethylenediamine tetraacetate. The resulting sheets were silver-white. After ensuing passivation in a conventional bath, as named in Example 1, without an intermediary treatment, these sheets were enameled without exhibiting any flaws or faults.

Example 5 The following Tables 1 and 2, respectively, show results obtained by treatments under the same conditions but using the indicated pretreatment solutions.

In carrying out these comparative tests, cold rolled steel sheets were first treated with an alkaline cleanser at 80 C., rinsed and then etched at C. As etching fluids a 10% aqueous HCl (Process A) or a 10% aqueous sulfuric acid solution (Process B) were employed. In both instances, the etching fluids also contained 4% iron in the form of ferrochloride or ferrosulfate, respectively, and a trace of dibenzylsulfoxide. After etching, the sheets were rinsed twice with water and then treated at 20 C. with the pretreatment solutions named in the tables. The ensuing passivation was effected at C. with a 0.3% aqueous soda solution, and the sheets then were dried at C. The treatment time in each bath was 5 minutes, the intervening dwelling times in the air 1 minute.

In the Tables 1 and 2, the following abbreviations were used:

EDTA: Tetrasodiumethylenediamine tetraacetate.

Acetophosphonate: Acylation products of phosphorous acid with acetyl chloride in the form of their sodium salts (produced according to JACS 34, 492499).

Polyalkanol: N,N,N,N tetrakis (2-hydroxypropyl)- ethylenediamine.

Partial or full substitution of soda, potassium hydroxide, potassium carbonate or mixtures thereof for NaOH in Table 2 led to the same results.

TABLE 1 Pretreatment Solution (Aqueous) 8% aOH 8% Sodium-potassium tartrate.

NaOH }Brown oxidic sodium gluconate" a dium glueonate 0 a0 EDTA (22.5% solution) I: mg I I Process A Process B }Strongly oxidle brown sheet surface.

sheet surface.

Slightly oxidic sheet surface.

J sheet surface. I .do

l l Do. l

do Brown oxidie sheet surface.

. do D0.

TABLE 2 Pretreatment Solution (Aqueous) Process A Process B surface.

0.8% N a H 0.8% triethanolamin 0.4% nitriloacetic acid.

Silver-white metal Silver-white metal surface.

The percentages given in Tables 1 and 2 are weight percent calculated on the total solution.

We claim as our invention:

1. A- process for the treatment of iron and steel sheet, after acid etching and prior to passivation preparatory to vitreous enameling, for the purpose of total removal of iron salts formed, which comprises exposing said sheet to a 1 to percent by Weight aqueous solution of a mixture consisting essentially of an alkali, selected from the group consisting of sodium hydroxide, potassium hydroxide, soda, potassium carbonate and combinations thereof, with an aliphatic alkanolamine having an alkyl radical of 2 to 4 carbon atoms and at least 3 alkanol groups in its molecule, for a period of 1 to minutes and at a temperature ranging from 10 to 90 C.; said solution having a pH above 10; the alkali constituting more than 25 and up to 50 percent by weight and the alkanolamine the remainder of said mixture.

2. The process as defined in claim 1, wherein said metal sheet is immersed in said solution.

3. The process as defined in claim 1, wherein said metal sheet is sprayed with said solution.

4. A process for the treatment of iron and steel sheet, after acid etching and prior to passivation preparatory to vitreous enameling, for the purpose of total removal of iron salts formed, which comprises exposing said sheet to a 1 to 5 weight percent aqueous solution of a mixture consisting essentially of a mixture of an alkali, selected from the group consisting of sodium hydroxide, potassium hydroxide, soda, potassium carbonate and combinations thereof, with an aliphatic alkanolamine having an alkyl radical of 2 to 4 carbon atoms and at least three alkanol groups in its molecule, and a compound selected from the group consisting of nitriloacetic acid, ethylenediaminetriacetic acid, ethylenediaminotet-raacetic acid and their water-soluble salts, for a period of 2 to 10 minutes and at a temperature ranging substantially from to 60 C.; the alkali constituting more than and up to 50 weight percent; the compound up to 20 weight percent, based upon the total amount of the compound taken together with said alkali and said alkanolamine; and the alkanolamine the remainder.

References Cited UNITED STATES PATENTS 2,836,566 5/1958 Duncan 134-2 X 2,932,584 4/1960 Hubbell et al 117-53 3,067,070 12/ 1962 Loucks. 3,132,975 4/1964 Freud. 3,154,438 10/1964 Keller et a1 148-615 FOREIGN PATENTS 821,094 9/1959 Great Britain.

MORRIS o. WOLK, Primary Examiner.

JOSEPH SCOVRONEK, Examiner. I. Z ATARGA, Assistant Examiner, 

